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   难度：Medium
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   <h1 class="question_title">
    47. Permutations II
   </h1>
   <p>
    Given a collection of numbers that might contain duplicates, return all possible unique permutations.
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   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> [1,1,2]
<strong>Output:</strong>
[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]
</pre>
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   <h1 class="question_title">
    47. 全排列 II
   </h1>
   <p>
    给定一个可包含重复数字的序列，返回所有不重复的全排列。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong> [1,1,2]
<strong>输出:</strong>
[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]</pre>
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